<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
    <style>
    html{
        height: 100%;
    }
    body{
        height: 100%;
        background-image: url(timg.jpg);
        background-size: cover;
    }
    #testDiv{
        width: 300px;
        height: 20px;
        position: relative;
        top: 100px;
        left: 50vw;
        
        
        text-align: center;
    }
    </style>
<body>
    
</body>
</html>
<script>
    var divEle = document.createElement("div")
    document.body.appendChild(divEle);
    divEle.innerHTML = "倒计时还有90分钟00秒"
    divEle.id = "testDiv";
    var time = 90*60;
    function daoJiShi(){
        time--;
        var minute = Math.floor(time/60);
        var second =time%60;
        minute = minute<10?"0"+minute:minute;
        second = second<10?"0"+second:second;
        divEle.innerHTML = "倒计时还有"+minute+"分钟"+second+"秒";

        setTimeout(daoJiShi,1000)
    }
    setTimeout(daoJiShi,1000)
</script>